In an electrophotographic apparatus such as a copying machine and laser printer, it sometimes happens that the surface potential of a photoreceptor greatly changes according to the environmental change such as a temperature change. For instance, when the photoreceptor has an OPC (Organic Photoconductive Conductor), since the mobility of optical carriers has the temperature dependency, the surface potential drops down by about 100 V under a low temperature circumstance compared to that of the normal temperatures, thereby causing the occurrence of a residual potential. Therefore, the toners moved to white parts of the copying picture image, thereby causing the fog.
There is the following tendency. More specifically, when the copying and printing operations are repeatedly carried out, a mechanical stress such as polishing due to a cleaning blade is accumulated so as to reduce the film thickness of the photoreceptor layer, thereby resulting in that the surface potential gradually drops. The occurrence of such change in the surface potential gives a great affection such as lowering of the density on the picture quality of formed picture image.
In contrast, the developer sensitive to a humidity change because of its powder. In general, when it is low humidity, the developer has a high electrical resistance, thereby resulting in that the frictionally charged toner has a strong charge holding capability. As a result, the charged amount of the toner increases, thereby changing the picture quality. Additionally, the developer deteriorates due to the repeated using thereof, thereby causing the great change of the picture quality.
In order to compensate the foregoing deficiencies, the conventional electrophotographic apparatus stabilizes the picture quality by measuring the electrostatic latent image formed on the photoreceptor so as to control the forming of electrostatic latent image in accordance with the measured results (see, for example, the Japanese examined patent publication No. 61-29502/1986).
There is another example which compensates the foregoing deficiencies. Such example is provided with means for detecting the optical density of the toner image on the photoreceptor so as to detect the change of quality and control the electrophotographic process in accordance with the detected results, so that the optimum picture quality can be obtained according to the example, a plurality of square toner patches of about 30 mm.times.30 mm as the toner image are provided on the photoreceptor so that each toner patch has a different density from other toner patches, thereby individually detecting the optical density thereof.
The following description deals with the process of detection of the optical density in accordance with the formation of the toner patches with reference to a time chart of FIGS. 15(a) through 15(f). The drum-type photoreceptor is charged on respective different positions by different grid voltages -500 V, -400 V, and -300 V in this order (see FIG. 15(a)). A copy lamp is turned off with respect to the charged area, so that the, exposure operation is not carried out (see FIG. 15(b)), and during the period a blank lamp is as shown in FIG. 15(c).
Therefore, electrical charges due to the grid voltages remain in the charged area as they are. Three toner patches (see FIG. 15(e)) having different density are formed by developing the charged area with a constant developing bias voltage of -200 V (see FIG. 15(b)), and each density of the toner patches is detected by an optical sensor in accordance with the detecting timing of FIG. 15(f). Thereafter, the grid voltages, the developing bias voltage, and other factors are controlled in accordance with each detected density of the toner patches so as to correct the picture quality.
After the control, the photoreceptor is charged by a constant grid voltage of -700 V. Then, the charged area of the photoreceptor is exposed by the copy lamp having respective applying voltages 60 V, 65 V, and 70 V. Three toner patches having different density are formed by developing the charged area with a constant developing bias voltage of -200 V, and each density of the toner patches is detected by an optical sensor in the foregoing manner. Then, the applying voltage of the copy lamp is controlled so as to correct the picture quality.
Note that it requires about 2 seconds to control the process in accordance with the detected optical density of the six toner patches upon formation of the toner patches. Accordingly, the process control based on the toner patches is carried out before and after (1) the turn-on operation of the main power of the copying machine and (2) the copying operation, so as not to bring any troubles during the copying operation.
However, the timing of process control based on the toner patches is only made during the turn-on operation of the main power of the copying machine. So, since the change of circumstances such as the temperature rise of 10.degree. C. to 15.degree. C. inside the copying machine occurs until performing the copying operation, the conventional apparatus presents the problem that the accuracy of the process control deteriorates.
In order to avoid the foregoing problem, when the timing is made before the copying operation, the job efficiency during the copying operation is reduced, thereby resulting in that the claims of its market. In contrast, when the timing is made after the copying operation, the density correction of picture image is not carried out until the copying operation finishes, thereby presenting the problem, which is similar to the case where the timing is made on the power-on, that the accuracy of the process control deteriorates.